A monorail type conveying apparatus is advantageous, being capable of making effective use of space; however, it has been conventionally driven by a rotary type motor.
Typically, such a conveying apparatus comprises, as shown in U.S. Pat. No. 4,480,157, a pair of carriers each having a support wheel adapted to roll on the monorail and a plurality of guide rollers adapted to engage both sides of the monorail at the upper and lower portions of the monorail, and at the front and rear positions of the conveying apparatus. A load bar interconnects the carriers beneath the monorail, and electric current feeding rails are disposed along one side of the monorail engaged by collectors provided on one of the carriers.
However, with a rotary type motor used as a driving source, the conventional monorail apparatus has a drawback that it requires a complicated transmission mechanism as well as a speed reducing mechanism with a resulting heavy weight.
Consequently, use of a linear induction motor (hereinafter referred to simply as "linear motor") has recently been proposed because of its large starting propulsive force, acceleration/deceleration (braking) and capability of doing away with a transmission mechanism as well as speed reducing mechanism.
With linear motors, it has been conventional to install a secondary conductor (non-energized member) on the upper surface or side face of a monorail; and the primary conductor member (power feed member) installed in a position opposing the secondary conductor.
It is to be noted, however, that a linear motor has a characteristic that it generates not only a propulsive force but an attracting force between the primary and the secondary members. Thus, if a secondary conductor is provided on the upper surface of the monorail, attracting force is applied to the running wheels of the conveying apparatus, increasing the force of the conveying apparatus on the monorail with a resultant increase in rolling friction. Consequently, a smooth running of the wheels is impeded, thereby obstructing an accurate control of operation of the monorail conveying apparatus.
One way to solve this problem is, of course, to increase the driving force of the linear motor. However, the weight of a linear motor increases in proportion to its propulsion force, which thereby increases the rolling friction.
Therefore, it has been conventional to set the propulsion force of the linear motor seeking a compromise between the amount of the rolling friction and the total weight of the conveying apparatus.
In addition, it has been a problem that as a secondary conductor, an aluminum plate lined with an iron plate is required, since the monorail structure is usually made of concrete.